This invention relates to a tone signal processing device performing controls such as a tone color control of a tone or a waveform interpolation operation by utilizing a digital filter operation.
As a tone signal processing device using a digital filter, there are prior art devices disclosed in Japanese Preliminary Patent Publication No. 62-127899 corresponding to U.S. Pat. No. 4,841,828 and other publications. In these devices, plural tone signal sample data are sequentially delayed in a delay register having multiple delay stages which correspond to filter coefficient orders in one-to-one relationship and filter coefficients corresponding to respective orders are supplied in response to the delayed outputs on a time shared basis to perform a filter operation. It is also disclosed that a filter operation is performed for tone signals of plural channels on a time shared basis. In these devices, however, a filter operation circuit of one channel cannot be used for all channels on a time shared basis, and consequently there are provided filter operation circuits of plural channels.
In these prior art devices, operation must be made with respect to all filter orders with the result that the amount of operation and the number of delay stages become enormous. For overcoming this problem, Japanese Preliminary Patent Publication No. 63-168695 corresponding to U.S. Pat. Ser. No. 07/139,659 proposes a simplified filter operation resembling interpolation. According to the proposed filter operation, a filter operation of m orders is made with respect to tone signal sample data of n samples (n&lt;m). More specifically, while tone signal sample data is sequentially generated in correspondence to an integer section of an address signal, n filter coefficients are selected from among filter coefficients of m orders in correspondence to the value of a decimal section of the address signal (the combination of orders of the selected filter coefficients differs depending upon the value of the decimal section of the address signal) and then, by operating tone signal sample data of n samples with the selected n filter coefficients, a filter operation of m orders is substantially performed. It is also disclosed that a filter operation is made with respect to tone signals of plural channels. In this case, a filter operation circuit employs a hardware circuit which is common to all channels and filter coefficient multiplication for n orders is made for each channel on a time shared basis. In this device, however, a delay circuit is not provided for providing tone signal sample data of n samples; instead, in performing the filter coefficient multiplication for n orders, tone signal sample data for n samples are read on a time shared basis from a waveform memory at a multiplication timing of each order.
In this Japanese Preliminary Patent Publication No. 63-168695, it is disclosed that, as tone signal sample data of n samples, data of n addresses stored in a waveform memory, rather than generated data of fixed n sampling periods, is used. This signifies that a simple filter operation according to a constant sampling period is not made but the sampling period of the filter operation changes with the tone pitch of a tone and thereby a waveform interpolation operation over n samples is made and data of one sample is formed as a result of this interpolation operation.
In a case where a filter operation is to be made with respect to tone signals of plural channels, it is readily conceivable to employ a common filter operation circuit for plural channels on a time shared basis. In such a case, however, because coefficient operation for each order for one channel is made on a time shared basis by using a common operation circuit, the number of time division slots will become extremely large when the number of filter orders is large (the time division slot number equal to the number of orders multiplied by the number of channels is required), as long as the operation is made in a normal manner. Therefore, a time division operation at an extremely high rate is required for maintaining a necessary sampling frequency. For this reason, the above mentioned Japanese Preliminary Patent Publication No. 62-127899 (U.S. Pat. No. 4,841,828) provides filter operation circuits of plural channels.
In the above mentioned Japanese Preliminary Patent Publication No. 63-168695, such an arrangement is provided as to carry out a filter operation of m orders through operation of n coefficients which are much fewer in number than m and, accordingly, the number of operation time slots for one channel is reduced and the employment of a common filter operation circuit for plural channels on a time shared basis can be made easily. In this prior patent application, however, supply of tone signal data of n samples is made not by provision of a delay circuit but by reading out tone signal sample data of n samples from a waveform memory on a time shared basis at a multiplication timing of each order, and this resultantly poses the problem that the time division operation timing for each order is undesirably limited by the access speed of the waveform memory. Besides, since relation between the channel time division timing and time division multiplication timing of coefficient of each order is relatively simple (i.e., time slots for one channel are divided further by the number of time division time slots of coefficient of each order), there arises the inconvenience that, as in the prior art devices, a time division operation at an extremely high rate is required for maintaining necessary sampling frequency when the number of orders is large.